Integrated circuits such as processors and other similar devices are operating at much greater speeds to perform an ever increasing number of operations each second. Many of these integrated circuits are placed on printed circuit boards or other similar structures and are in electrical communication with many different electrical components and other integrated circuits resident on the same printed circuit board. In order to facilitate communication between the integrated circuits and the several other electronic components, the integrated circuit contacts electrical conductors on a printed circuit board through pads that are often arranged in a dense grid or array on the printed circuit board.
Oftentimes, it is necessary to test the operation of such new integrated circuits after they are fabricated either to test prototypes or to diagnose problems experienced with the integrated circuits. In particular, generally one or more of the pads in the array into which the integrated circuit is inserted are probed to access the signal thereon so that the signal can be transmitted to a logic analyzer or oscilloscope. The fact that the pins of the integrated circuits and corresponding pads on the printed circuit board are arranged in a dense array make such testing difficult to accomplish in light of the high frequency operation of such integrated circuits.
To explain further, a typical printed circuit board includes several groups of signal conductors that run between various components on the board. When a probe conductor is joined to one of the pads, a very small capacitance between the probe conductor and the signal conductors on the order of picofarads presents an undesirable load impedance on the pads. In particular, at low frequencies, this impedance is acceptably high. However, at very high signal frequencies on the order of hundreds of megahertz, the impedance presented by such a capacitance will drop, resulting in extraneous loading on the conductors between the integrated circuits.
In addition, a similar small capacitance may exist between the probe conductor and the remaining pads, or aggressor pads in the pad array. At high frequencies, these capacitances present a low impedance which results in cross-talk between the aggressor pads and the probe conductor.
This extraneous loading and cross-talk results in distortion of the signal on the pins of the integrated circuit that causes error to the data represented by the transmitted signals. Consequently, the ability to test the integrated circuit is hampered by the use of the probe itself.